Investigating the influence of thermal Post-Processing on the Mode I and Mode II fatigue fracture resistance of additively manufactured carbon Fiber composites

• Thermal post-processing improved fatigue resistance in AM CF/PA composites. • Mode I and Mode II prefactors dropped by ≈ 87 % and ≈ 76 %, respectively. • Fatigue mechanisms unchanged; improvements reflect higher effective thresholds. • Normalisation showed 36 % of Mode I and 27 % of Mode II gains were toughness-driven. • CF20 simulations and SEM confirmed toughness- and friction-based fatigue gains. Interlaminar defects such as voids and incomplete fibre–matrix consolidation limit the fatigue performance of additively manufactured (AM) continuous-fibre composites. This study examines how thermal post-processing influences Mode I (DCB) and Mode II (ENF) fatigue crack-growth behaviour in 3D-printed CF/PA laminates. Across all load levels, post-processed specimens showed substantially improved fatigue resistance, requiring approximately 40–50% higher energy-release rates to reach the same crack-growth rates as the as-printed material. Crack-length–versus-cycle curves further demonstrated that post-processed specimens endured significantly more cycles to reach equivalent crack lengths. Microscopy revealed distinct mechanisms underlying these improvements: void elimination and smoother crack planes in Mode I, and increased crack-face contact and frictional dissipation along shear-sliding surfaces in Mode II. Finite-element simulations using the CF20 fatigue cohesive model reproduced the experimental trends with minimal calibration. Overall, thermal post-processing provides a simple and effective pathway to enhance the fatigue durability of additively manufactured composite laminates.